This invention was developed under DOE Subcontract No. ZAX-8-17647-10.
This invention relates to the manufacture of photovoltaic solar cell modules and more particularly to provision of solar cell modules having an improved backskin.
A common form of solar cell module is made by interconnecting individually formed and separate solar cells, e.g., crystalline silicon solar cell, and then mechanically supporting and protecting the cells against environmental degradation by integrating the cells into a laminated solar cell module. The laminated modules usually comprise a stiff transparent protective front panel or sheet, and a rear panel or sheet typically called a xe2x80x9cbackskinxe2x80x9d. Disposed between the front and back sheets so as to form a sandwich arrangement are the interconnected solar cells and an encapsulant. A necessary requirement of the encapsulant (or at least that portion thereof that extends between the front sides of the cells and the transparent front panel) is that it be transparent to solar radiation. The typical mode of forming the laminated module is to assemble a sandwich comprising in order a transparent panel, e.g., a front panel made of glass or a transparent polymer, a front layer of at least one sheet of encapsulant, an array of solar cells interconnected by electrical conductors (with the front sides of the cells facing the transparent panel), a back layer of at least one sheet of encapsulant, a sheet of scrim to facilitate gas removal during the lamination process, and a backskin or back panel, and then bonding those components together under heat and pressure using a vacuum-type laminator. The back layer of encapsulant may be transparent or any other color, and prior art modules have been formed using a backskin consisting of a thermoplastic polymer, glass or some other material.
Although the lamination process seals the several layered components together throughout the full expanse of the module, it is common practice to apply a protective polymeric edge sealant to the module so as to assure that moisture will not penetrate the edge portion of the module. The polymeric edge sealant may be in the form of a strip of tape or a caulking-type compound. Another common practice is to provide the module with a perimeter frame, usually made of a metal like aluminum, to provide mechanical edge protection. The foregoing prior art techniques are disclosed or suggested in U.S. Pat. No. 5,741,370, issued Apr. 21, 1998 to Jack I. Hanoka for xe2x80x9cSolar Cell Modules With Improved Backskin And Methods For Forming Samexe2x80x9d. That patent also discloses the concept of eliminating the back layer of encapsulant and bonding a thermoplastic backskin directly to the interconnected solar cells.
Heretofore a large number of materials have been used or considered for use as the encapsulant in modules made up of individual silicon solar cells. Until at least about 1995, ethylene vinyl acetate copolymrer (commonly known as xe2x80x9cEVAxe2x80x9d) was considered the best encapsulant for modules comprising crystalline silicon solar cells. However, EVA has certain limitations: (1) it decomposes under sunlight, with the result that it discolors and gets progressively darker, and (2) its decomposition releases acetic acid which in turn promotes further degradation, particularly in the presence of oxygen and/or heat.
U.S. Pat. No. 5,478,402, issued Dec. 20, 1995 to J. Hanoka, discloses use of an ionomer as a cell encapsulant substitute for EVA. Relevant information contained in that patent is incorporated herein by reference. The use of ionomer as an encapsulant is further disclosed in U.S. Pat. No. 5,741,370, supra. The term xe2x80x9cionomerxe2x80x9d and the type of resins identified thereby are well known in the art, as evidenced by Richard W. Rees, xe2x80x9cIonic Bonding In Thermoplastic Resinsxe2x80x9d, DuPont Innovation, 1971,2(2), pp. 1-4, and Richard W. Rees, xe2x80x9cPhysical Properties And Structural Features Of Surlyn(copyright) lonomer Resinsxe2x80x9d, Polyelectrolytes, 1976, C, 177-197. Ionomers may be formed by partial neutralization of ethylene-methacrylic acid copolymers or ethylene-acrylic acid copolymers with organic bases having cations of elements from Groups I, II, or III of the Periodic Table, notably, sodium, zinc, aluminum, lithium, magnesium and barium. Surlyn(copyright) ionomers have been identified as copolymers of ethylene and methacrylic acid that typically have a melting point in the range of 83xc2x0-95xc2x0 C.
Although it is known to use a rear panel or backskin that is made of the same material as the front panel, a preferred and common practice is to make it of a different material, preferably a material that weights substantially less than glass. e.g., a material such as TEDLAR(copyright) (the trade name for a polyvinyl fluoride polymer made by E.I. DuPont de Nemeurs Co.). Heretofore a popular and widely used backskin material has been a TEDLAR/polyester/ethylene vinyl acetate laminate. However, TEDLAR and TEDLAR laminates are not totally impervious to moisture, and as a consequence over time the power output and/or the useful life of modules made with this kind of backskin material is reduced due to electrical shorting resulting from absorbed moisture.
U.S. Pat. No. 5,741,370, supra, suggests that manufacturing and module mounting costs could be reduced by using as the backskin material a thermoplastic olefin comprising a combination of two different ionomers, e.g., a sodium ionomer and a zinc second ionomer, with that combination being described as producing a synergistic effect which improves the water vapor barrier property of the backskin material over and above the barrier property of either of the individual ionomer components. Significantly the patent discloses shows use of an ionomer encapsulant with the dual ionomer backskin.
However, the water absorption characteristics of all sodium ionomers are not identical. The same is true, but to a lesser extent, of zinc ionomers. More importantly, the water absorption characteristics of zinc ionomers tend to be substantially less, than those of sodium ionomers. Significantly modules made using sodium based ionomers as encapsulants, as taught by U.S. Pat. No. 5,478,402, tend to degrade with time, often in a matter of months, with portions of the encapsulant changing color. This discoloration, which occurs at multiple points along the length and breadth of the module, reduces the ionomer""s light transmissibility, thereby lowering the module""s energy conversion efficiency and power output, as well as rendering it less appealing from an aesthetic viewpoint. Degradation of the ionomer destroys the electrical insulation resistance, causing electrolytic corrosion plus a loss of required safety provisions. Such sodium ionomer degradation is known to result from moisture absorption, and also from another cause related to the solar cell interconnections.
As disclosed in copending U.S. patent application Ser. No. 10/35,107, pending filed Dec. 27, 2001 by R. C. Gonsiorawski for xe2x80x9cEncapsulated Photovoltaic Modules And Method Of Manufacturing Samexe2x80x9d, the teachings of which are incorporated herein by reference, sodium ionomer degradation may be induced by solder flux residues. It should be noted that the conductors (tabbing) used to interconnect individual solar cells are secured in place by a solder. The solder may be applied separately or the conductors may be pre-tined, i.e., provided with a solder layer, to facilitate soldering. In both cases, the solder or soldering process includes a flux composition for removing oxide films and ensuring adequate wetting of surfaces. Commonly it is preferred to use pre-tinned conductors which also have been coated with a suitable flux composition. The fluxes commonly comprise an inorganic or organic acid or acid salt, e.g., a carboxylate or a benzoate compound such as the one sold under the name xe2x80x9cPentoatexe2x80x9d. Although the flux compositions are designed to be eliminated by vaporization directly or via decomposition during the soldering process, in practice some flux residue may continue to exist at the soldered connection points in contact with the surrounding ionomer encapsulant. Sodium ionomers, also known as sodium-based ionomers, are reactive to acid, with the result that acid reaction discolorations tend to occur in those portions of the sodium ionomer that come in contact with the flux residues. Because the flux residue tends to be present in a small amount and scattered throughout the module, its deleterious affect on the encapsulant may not be immediate or immediately visible. However, over time, particularly under relatively high ambient temperatures, the flux-induced degradation of the sodium ionomer can progress sufficiently to adversely affect the power output and stability of the module. By way of example, and as noted in said copending application Ser. No. 10/035,107, flux residues have been determined to be a factor in failure of modules wherein the encapsulant was the SURLYN(copyright) 1601 ionomer recommended in U.S. Pat. No. 5,478,402, supra.
The invention described in said copending application Ser. No. 10/035,107 overcomes the problem of flux-induced ionomer degradation by using an encapsulant in the form of a zinc ionomer. Zinc ionomers exist that are available in sheet form, transparent, stable and capable of heat bonding to adjacent components of a solar cell module. Zinc ionomers tend to absorb water less readily than sodium ionomers. Such zinc-based ionomers are exemplified by two DuPont products identified as (copyright) 1705-1 and (copyright) 1706. These polymers have excellent optical properties and high hot tack strength. The exact chemical composition of these materials is not known, but they are believed to be produced by adding a salt containing zinc cations to a copolymer of ethylene-methacrylic acid, or to a copolymer of ethylene-acrylic acid, and subjecting that composition to acid neutralization, resulting in the formation of ion clusters within the resulting polymer matrix. The (copyright) 1705-1 ionomer material has a water absorption of 0.3% wt. % in comparison with the sodium based (copyright) 1601 and EVA which have water absorptions of 3.0 wt. % and 0.7% wt. % respectively. Further, the zinc based ionomers are substantially inert to acid flux residues. As a consequence, modules made using the zinc (copyright) 1705-1 ionomer as encapsulant and a TEDLAR(copyright) backskin were found to pass stress tests of 1000 hours at 85% RH/85xc2x0 C. damp heat as well as the humidity-freeze cycling (85/85 to 0-40) for 20 cycles without decreased electrical photovoltaic performance while fully satisfying the safety criteria of the wet and dry high voltage withstand tests at 3600 volts as well as the insulation resistance criteria measured at 500 volts.
Nevertheless it is desirable to provide modules that are capable of withstanding stress tests for substantially more than standard 1000 hours. It also is desirable to provide a backskin material and a method of forming laminated solar cell modules incorporating same. In this connection, it should be noted that the conductors that interconnect the solar cells commonly are arranged to form stress relief loops to compensate for expansion and contraction caused by temperature changes. Those loops need to be encapsulated with the cells. However, when a polymeric backskin is used, are must be taken to make certain that the stress loops will not pierce the backskin when the several layers are compressed under heat to form the laminated module. Penetration of the backskin by one or more stress loops will promote early failure of the module, e.g., by short-circuiting resulting from ingress of moisture at the point(s) of stress loop penetration the backskin. The present invention recognizes that the thermoplastic backskin should have a melting point higher than the encapsulant, and capitalizes on the fact that the likelihood of backskin penetration by components of a solar cell module, during or as a consequence of the lamination process, increases the closer that the melting point of the thermoplastic backskin is to the melting point of the encapsulant.
One primary object of this invention is to improve upon the backskin component of solar cell modules.
Another primary object of this invention is to provide solar cell modules characterized by an improved backskin.
Another object is to provide an improved method of making solar cell modules characterized in that the modules embody improved backskins.
Still another object is to provide modules characterized by backskins made of a material that has a high dielectric strength and extends the useful life of the modules.
A further object is to provide modules of interconnected crystalline silicon solar cells that withstand degradation substantially better than modules made with backskins of TEDLAR(copyright), TEDLAR(copyright) laminates, or a combination of two ionomers.
A more specific object is to a backskin material that bonds readily to the ionomer encapsulant and associated components of a laminated solar cell module with little risk of it being pierced by mechanical components of the module during or as a result of the laminating process.
A further object is to provide laminated solar cell modules having a backskin that melts at a relatively high temperature and is sufficiently tough to resist being pierced or penetrated by mechanical components of the module as a consequence of the heat and pressure applied during the laminating process.
The foregoing objects and other objects rendered obvious from the following detailed description are achieved by using an ionomer/nylon alloy as the backskin material. That novel backskin material has a number of advantages that are described in or rendered obvious by the following detailed description of the invention.